FIG. 1 illustrates an aircraft driven by counter-rotating, pusher propellers 1A and 1F. The propeller blades are swept as shown in FIGS. 6 and 13 to allow flight speeds up to mach 0.85. It has been found that the arrangement shown creates several unique sources of vibratory excitation which disturb the propeller blades. For example, the wake 2 shed by the wing 3 presents a discontinuity through which the propeller blades must pass, providing one source of excitation. A second source of excitation arises when the aircraft has a high angle of attack, as occurs during takeoff, climb and approach. At this time, the fuselage sheds a vortex 6 which enters the propellers 1A and 1F. The vortex 6 disturbs the propeller blades first when they pass through a first edge 8 of the vortex and also when they leave, passing through edge 10. In addition, high air disturbance is encountered during pitch, yaw and roll maneuvers.
The blades themselves have a natural resonant frequency in the range of ten to seventy hertz. Given that the propellers rotate at about twenty revolutions per second, the one-per-revolution and two-per-revolution excitations caused by the wake 2 and the vortex 6 provide a stimulus within this response range. That is, the wake provides twenty excitations per second per blade, and the vortex provides forty excitations per second.
In previous designs, the resonant frequency of aircraft fan blades (not propeller blades) has been reduced by the use of so-called pinned root mounting systems. One example is the pinned root system found on the TF 34 engine which is sold by the General Electric Company. This type of mounting is illustrated in FIG. 2. A hinge 12 is fastened to the base of each fan blade 14, thus allowing the blade to rotate between dashed positions 16 and 18. There are several significant features in this arrangement. One, as shown in FIG. 3, the holes 20 and 22 in the hinge are larger in diameter 24 than the diameter 26 of pin 28. As a result, when blade 14 rotates about the pin axis 63, the pin rolls (rather than slides) to a new position as shown in FIG. 3A. There is little or no rubbing of pin 28 in holes 20 and 22. Reference mark 30 indicates that pin 28 rolls and does not slide.
Two, the pinned root system just described has been used on a turbofan engine in which the pitch of the fan blades is fixed. That is, as shown in FIG. 4, which is a view of one blade 14 in FIG. 2 taken along line 4, there is no rotation of the fan blades from solid position 14 to phantom position 14A, indicated by arrow 33. To repeat, such rotation about pitch axis 29 in FIG. 2, or pitch change, is absent.
The absence of pitch change, coupled with high blade centrifugal forces, tolerates the use of the rolling, loose pin shown in FIG. 3. However, when a pitch-changing blade is used at low rotational speeds, the loose pin mount causes the problem which is shown in exaggerated form in FIG. 5. The aerodynamic forces and the centrifugal forces acting on the blade tend to drive the pin 28 into a skewed position as shown. Such skewing is undesirable at least for the reason that the angle of attack of the blade becomes somewhat decoupled from the position of lower hinge half 57 in FIG. 2.
A third feature is that the hinge pin 28 is located outside, and not within, the fan stream portion indicated by streamlines 31. That is, the hinge pin is located below the blade platform 58 in FIG. 2, behind the spinner 59. In such a location, the pin is surrounded by air at a low (approximately ambient) temperature.
A fourth feature is that the pin is approximately parallel with the axis of rotation 66 of the fan. This causes the leading edge 61 of the blade 14 to remain in the same radial plane when deflection into phantom position 16 occurs.